Interior Gateway Protocols Versus Exterior Gateway Protocols
Even though many unicast routing protocols were developed in the early days of the ARPANET (the
predecessor to the Internet), Routing Information Protocol (RIP) emerged as the most popular. Many
independent networks that were created at govern-ment research institutions and universities as a
result of the remarkable success of the ARPANET also adopted RIP for dynamic routing operations.
The evolution of the ARPANET into the Internet required the numerous island networks to be
interconnected using a more robust routing protocol. The Exterior Gateway Protocol (EGP) was
selected for this purpose. EGP provided an efficient mechanism for routing among the various RIP
domains. Therefore, RIP and EGP were optimized for distinct functions in the network based on their
capabilities. RIP was used for intradomain routing, and EGP was used for interdomain routing. EGP
later morphed into the Border Gateway Protocol (BGP), and other more robust protocols optimized
for intradomain routing emerged in place of RIP. In particular, the Open Shortest Path First (OSPF)
Protocol was developed in the Internet Engineering Task Force to provide capabilities that RIP lacked,
such as more intelligent routing metrics, faster convergence, and operation in classless
environments. So, here we are again with yet another classification of routing protocols: interior
gateway routing protocols (for intradomain routing) and exterior gateway protocols (for interdomain
Figure 1-9 shows two routing domains, AS 65001 and AS 65002, and an overlapping (shaded) region
depicting the interconnection between border routers from each domain. In more current routing
terminology, a routing domain also is referred to as an autonomous system. An autonomous system
is an independent routing domain under the control of a single administrative authority.
Figure 1-9. Intradomain and Interdomain Routing
As noted before, an exterior gateway protocol provides the capability for sharing routing information
between the two domains. Currently at version 4, BGP is the only IP inter-domain protocol that is
used for interconnecting the numerous autonomous systems in the global Internet. An interior
gateway protocol provides routing intelligence within an autonomous system. Each of the
autonomous systems in the Internet can run any suitable IGP. With the exception of EGP (the
obsolete routing protocol) and BGP, all the other unicast protocols mentioned so far—IGRP, EIGRP,
RIP, OSPF, and IS-IS—are IGPs (see Table 1-4).
Table 1-4. IGP and EGP Classification
The Interior Gateway Routing Protocol (IGRP) was invented by Cisco Systems to offer better metrics
than the simple hop count supported by RIP. IGRP introduced a composite metric that consists of
• Maximum transmission unit (MTU)
Cisco evolved IGRP into the Enhanced Interior Gateway Routing Protocol (EIGRP). EIGRP provides
faster convergence relative to IGRP by using backup routes, referred to as feasible successor routes,
that are readily installed in the routing table when a preferred route is lost. Unlike IGRP, EIGRP
OSPF and IS-IS are both popular IGPs used in very large IP networks. IS-IS originally was designed
as a routing protocol for the Connectionless Network Protocol (CLNP) but later was adapted to route
IP about the same time that the Open Shortest Path First (OSPF) proto-col was being standardized in
the Internet Engineering Task Force (IETF). OSPF and IS-IS are both link-state protocols, whereas
RIP, IGRP, and EIGRP are distance vector protocols.
Also, OSPF and IS-IS are link-state protocols that use the shortest path first (SPF) algorithm (named
after Dijkstra) for route computation, making them converge relatively fast in re-sponse to network
Both protocols also support a two-level hierarchical routing architecture. OSPF and IS-IS are very
similar protocols with almost identical capabilities. However, they have some architectural differences
that are beyond the scope of this book.
An interesting point to note, however, is that OSPF was designed entirely for IP only, and OSPF
packets are encapsulated in IP packets. In contrast, IS-IS was designed for CLNP and was adapted to
support IP additionally. IS-IS packets are not encapsulated in IP packets but rather directly by the
data link protocol.
The next section of this chapter looks at yet another routing protocol classification: distance vector
and link-state protocols.